Yes, it is possible to write below the signature, but is a few extra steps as currently designed.
To write under the signature, switch to "code view", Scroll down past your signature, add 2 periods after the last DIV as plain text, not in another DIV, then switch back to regular HTML view.
now you can replace those 2 periods with whatever text you want to type.

Derek, It would be nice if Smarter Mail did insert 1 blank regular line after the signature for this type of purpose. I also noticed that in replies, If i did not have the 2 extra periods there, there is no way to select the signature for deletion as it seems to treat it as part of a block of the next email or something. Sort of strange / cumbersome to try to delete just the signature if I have a long chain of back and forth responses if those 2 dots (an extra line) is not present after my block of signature.
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